1. Field of the Invention
The present invention relates to a hologram recording material suitable for volume hologram recording and a hologram recording medium including a hologram recording layer composed of the hologram recording material.
2. Description of the Related Art
Various volume hologram recording materials that use photosensitive resins have been proposed, and some of them have been put to practical use. Photosensitive resins are generally called “photopolymers”. Unlike known silver halide photosensitive materials, hologram recording materials that use photopolymers have a significant advantage in that no development treatment is required.
A typical photopolymer material for hologram recording is composed of, for example, a photopolymerizable monomer (I), a polymeric binder (II), a plasticizer (III), and a photopolymerization initiator (IV). By irradiating a hologram recording material formed of the photopolymer material in a layer structure with recording light on which an interference pattern is superimposed, the interference pattern is recorded in the recording material as a refractive index difference (refractive index modulation: Δn). The reaction mechanism in the recording material is considered to be as follows. That is, the photopolymerization initiator (IV) is cleaved in bright portions of the interference pattern through exposure, which triggers the polymerization of the photopolymerizable monomer (I) present near the photopolymerization initiator (IV). This decreases the concentration of an unreacted monomer in the bright portions, and consequently a monomer concentration gradient is generated between the bright portions and dark portions. To compensate for the generated concentration gradient, the unreacted monomer diffuses from the dark portions to the bright portions and thus the polymerization reaction of the monomer further proceeds in the bright portions. As a result, a large amount of a polymer of the photopolymerizable monomer (I) is present in the bright portions. Herein, by selecting the above-described components so that a refractive index difference between the photopolymerizable monomer (I) (and the polymer thereof) and the other components becomes large, a pattern of the bright and dark portions is recorded as a refractive index modulation (Δn).
By superimposing, on the interference pattern, reflected light including phase information from a certain subject or three-dimensional image information that is made using a personal computer and output to a spatial light modulator (SLM), a hologram including the three-dimensional image information recorded thereon can be obtained. Furthermore, by superimposing two-dimensional digital data on the interference pattern, a data recording medium that can record and reproduce a large amount of digital data at high speed can be provided. In principle, the refractive index modulation (Δn) is a performance indicator intrinsic to a recording material and is desirably as high as possible. In the case where the recording material is used for a hologram for image recording, the contrast increases with increasing Δn. In the case where the recording material is used for a data recording medium, the data storage capacity increases with increasing Δn.
In particular, when the hologram recording material is used for image recording, it is important to increase the refractive index modulation due to the reason below.
In general, the relationship between refractive index modulation and diffraction spectrum of a volume hologram is described by a coupled wave theory of Kogelnik (H. Kogelnik, “Coupled wave theory for thick hologram gratings”, Bell Syst. Tech. J., Vol. 48(9), pp. 2909 to 2947, (1969)). According to this theory, the relationship between the refractive index modulation of a refractive index grating formed in a recording material and the diffraction efficiency and the relationship between the refractive index modulation and the full width at half maximum (FWHM) of a diffraction spectrum can be estimated under certain recording conditions (recording wavelength, the incident angles of two plane waves that form interference fringes, the thickness of a recording material, the average refractive index of a recording material).
In a reflection-type hologram used for image recording, the diffraction efficiency (the peak value of a diffraction spectrum) rapidly increases to 100% with an increase in a refractive index modulation (Δn) whereas the full width at half maximum substantially linearly increases with an increase in a refractive index modulation Δn. That is, a higher value of Δn is required to achieve both high diffraction efficiency and a large full width at half maximum.
When an image is recorded on a hologram recording material, the brightness of the image is considerably dependent on not only diffraction efficiency (reflectance) but also full width at half maximum. In general, the recorded hologram image is illuminated with white light such as sunlight or light from a white light-emitting diode (LED), and light in a wavelength range corresponding to the diffraction spectrum is diffracted (reflected). Consequently, an observer visually confirms the recorded hologram image.
It has been known that there are a hologram recording material containing, as a high-refractive-index component (or a low-refractive-index component), only a monomer that is active in radical polymerization; a hologram recording material containing, as a high-refractive-index component (or a low-refractive-index component), only a monomer that is active in cationic polymerization; and a hologram recording material containing, as a high-refractive-index component (or a low-refractive-index component), only a monomer that is active in anionic polymerization. In general, radical polymerization is advantageously used to obtain a recording material with high recording sensitivity because the reaction rate of radical polymerization is higher than that of ionic polymerization such as cationic polymerization or anionic polymerization. Therefore, most of publicly known hologram recording materials contain a radical polymerizable monomer as a high-refractive-index component. However, the life of a polymerization active species (radical) in radical polymerization is extremely short (about several nanoseconds to several milliseconds), and thus principal monomer diffusion occurs only during recording exposure. Therefore, a high refractive index modulation cannot be achieved.
Japanese Patent No. 2873126 (Patent Document 1) discloses, as a specific photopolymer material, a photosensitive composition for volume hologram recording including a cationic polymerizable compound (a) that is in a liquid state at room temperature, a radical polymerizable compound (b), a photoradical polymerization initiator system (c) that polymerizes the component (b) through exposure to laser light or coherent light having a certain wavelength, and a photocationic polymerization initiator system that has low photosensitivity to the light having the certain wavelength but polymerizes the component (a) through exposure to light having a wavelength different from the certain wavelength.
The average refractive index of the component (a) is lower than that of the component (b). In other words, the component (a) corresponds to the above-described plasticizer (III). By providing cationic polymerizability to a plasticizer that is in a liquid state at room temperature, the plasticizer is made to serve as a plasticizer that facilitates the diffusion of the photopolymerizable monomer (I) during recording. In addition, the plasticizer can be cured through cationic polymerization using ultraviolet light or the like after the recording. Therefore, a liquid component is not left in a recording film after irradiation with ultraviolet light (called “postcure”), which improves the stability of recording signals.
Japanese Unexamined Patent Application Publication No. 2001-282082 (Patent Document 2) discloses a hologram recording material composition including an allyl-based prepolymer (A), a radical polymerizable compound (B), a viscosity reducing agent (C), and a photopolymerization initiator (D). It is disclosed in paragraph [0083] that the viscosity reducing agent (C) is a compound (C1) that does not react with the allyl-based prepolymer (A) and/or the radical polymerizable compound (B) or a compound (C2) having a (meth)allyl group in its molecule. In Patent Document 2, the viscosity reducing agent (C) corresponds to the above-described plasticizer (III). In the viscosity reducing agent (C), the compound (C2) having a (meth)allyl group in its molecule is active in radical polymerization like the radical polymerizable compound (B). However, since the polymerization activity is lower than that of the component (B), the polymerization of the component (B) preferentially proceeds during recording. At this time, the component (C2) facilitates the diffusion of the component (B). After the polymerization of the component (B), the component (C) is polymerized. Therefore, a liquid component other than the component (C1) is not left in a recording film after exposure, which improves the stability of recording signals as in the case of Patent Document 1.
Japanese Patent No. 3370762 (Patent Document 3) discloses a hologram recording material that does not necessarily include the above-described plasticizer (III), the hologram recording material including a certain polymeric binder, an ethylenically unsaturated monomer, and a photopolymerization initiator. It is disclosed in paragraph [0045] that, in this hologram recording material, the refractive index modulation is increased by performing a heat treatment at 100° C. to 150° C. for 0.5 to 2 hours after hologram recording. The reason for this is unclear, but it is believed that part of the ethylenically unsaturated monomer that has not diffused into bright portions of interference fringes during recording preferentially diffuses into a region where the ethylenically unsaturated monomer has been localized during the recording by a driving force such as a chemical potential difference under heating conditions.
In the materials disclosed in Patent Documents 1 and 2, a hologram is recorded on the basis of the above-described typical recording principle for a photopolymer material. In other words, refractive index modulation is generated due to a decrease in monomer concentration in bright portions of interference fringes during recording exposure and the monomer diffusion that uses, as a driving force, the concentration gradient generated by the decrease in monomer concentration. Therefore, since almost no monomer diffusion occurs when recording exposure is not performed, it is difficult to achieve a high refractive index modulation.
In particular, in the material disclosed in Patent Document 2, part of the viscosity reducing agent (C) corresponding to the plasticizer (III) is active in radical polymerization like the radical polymerizable compound (B). Therefore, the part of the viscosity reducing agent (C) is also polymerized during recording together with the component (B) and thus taken in the bright portions of interference fringes. That is, since the part of the component (C) that should have been localized in dark portions of interference fringes is also present in the bright portions of interference fringes, it is difficult to achieve a high refractive index modulation.
In the material disclosed in Patent Document 3, the refractive index modulation is increased by performing a heat treatment after recording. This means that, even after recording exposure, monomer diffusion can occur under certain conditions. However, the heat treatment is performed under severe conditions such as a maximum temperature of about 150° C. and a maximum duration of about 2 hours. This poses problems in that the material of a substrate on which a recording material is formed is limited, and the heat treatment process is complicated.